It is well known to utilize a laser beam welding method for the butt-welding of thin metallic plate materials because the laser beam energy is able to be concentrated to a narrow range. In accordance with the butt-welding method for thin metallic plates by utilizing a laser beam, it is required to increase the density or concentration of the laser beam energy in accordance with an increase of the welding speed, but in a high speed welding method, adverse phenomena may be caused, such as, for example, humped protrusions, or so called humpings, which are discontinuously formed along the front side of the welded bead or the central portion of the welded portion continuously projects above the front surface of the welded plates while the rear side thereof is the recessed, which is so called undercut formation which is formed under certain welding conditions. In case such an adverse phenomenon progresses, the welded portion is broken so as to form a hole. In particular, for example, when a surface treated steel plate having a thickness of approximately 0.2 mm is welded at a welding speed of more than 30 m/min., such adverse phenomena will frequently occur.
In accordance with conventional laser beam welding method, is well known that the formation of a keyhole will occur due to the laser beam welding, and in the case of high speed welding, the keyhole does not extend directly downwardly but extends obliquely towards the downstream direction of the welding. For this reason, the welded portion protrudes upwardly and the undercut portion may also be formed upon the rear surface thereof or in other words the humping phenomenon may occur. Such a phenomenon remarkably occurs in a case where the laser beam is irradiated upon a flat plate or a so called bead-on plate. For example, this phenomenon usually occurs in the case of a laser beam migration speed of more than 20 m/min. with respect to a bead-on plate comprising a surface treated steel plate having a thickness of 0.2 mm. The reason why the humping phenomenon or formation of the protrusions occurs during the laser beam welding method in comparison with the butt welding method is based on the remarkable curving of the keyhole towards the downstream side of the welding direction.
In addition, a burr is liable to occur during the welding process because thin metallic plates to be welded together are generally cut by means of a slitter, and the burr formed at the butted portion may constitute an obstacle for the laser beam butt-welding process which will prevent attaining good welding conditions. Namely, the laser beam irradiated upon the butted portions of the thin metallic plates is reflected and the energy of the laser beam is dispersed therearound. Moreover, the burr having a relatively small mass will be immediately evaporated when the burr absorbs the laser beam energy and the evaporated metallic vapour may adversely prevent the complete penetration of the laser beam into the weld area, thus affecting the transmission efficiency of the laser beam. Since the lowering of such transmission efficiency of the laser beam is not made uniformly along the butted portion, of each plate the heating process step along the welded beads is not uniformly achieved, and therefore, this is the a significant problem for the laser beam welding.
U.S. Pat. No. 4,577,088 discloses a welding method for substantially eliminating the problems or disadvantages described above, and in accordance with this method, two members to be mutually butt-welded are inclined at predetermined angles so that the upper edges of the members are separated with respect to each other and pressure is applied to these members in the direction of the butted edges thereof so as to respectively achieve lineal contact of the members at the butted edge portions thereof. In this state, the laser beam is irradiated upon the butted portions in a direction inclined to the direction along which the butted ends of the members extend vertically. According to the welding method disclosed in this patent, the problems described hereinbefore can be somewhat resolved, and in this method, the burr of the thin metallic plate is positioned at a location away from the center of the irradiated laser beam, so that the problem regarding the existence of the burr may be resolved.
However, in the method disclosed in the U.S. Pat. No. 4,577,088, the members to be welded are butted at a predetermined open angle with respect to each other so as to obtain a lineal contact of the butted portions of the members to be welded, whereby discordance between the butted portions thereof may be easily caused, and accordingly, it is difficult to accurately and stably butt or mate the end portions of the members to be welded. In addition, the butting of the members is performed at a predetermined open angle, so that the butted surfaces after completion of the butt-welding is not made substantially flat and may be formed in a skewed manner.
Furthermore, the contacting portion and the portion near this contacting portion of the lower end portions of the butted portions of the members to be welded have relatively small masses, so that these portions are liable to develop holes as a result of evaporation within a short period of time when the laser beam is irradiated upon these portions such as in the case where sheets having extremely thin thicknesses are butt-welded.